1. Field of the Invention
The present invention relates generally to improved methods of preventing well fracture proppant flow-back, and more particularly, to improved methods of fracturing a subterranean zone and propping the fractures whereby proppant flow-back from the fractures is prevented.
2. Description of the Prior Art
Oil and gas wells are often stimulated by hydraulically fracturing subterranean producing zones penetrated thereby. In such hydraulic fracturing treatments, a viscous fracturing fluid is pumped into the zone to be fractured at a rate and pressure such that one or more fractures are formed and extended in the zone. A solid particulate material for propping the fractures open, referred to herein as "proppant," is suspended in a portion of the fracturing fluid so that the proppant is deposited in the fractures when the viscous fracturing fluid is caused to revert to a thin fluid and return to the surface. The proppant functions to prevent the fractures from closing whereby conductive channels are formed through which produced fluids can readily flow.
In order to prevent the subsequent flow-back of the proppant with fluids produced from the fractured zone, at least a portion of the proppant has heretofore been coated with a hardenable resin composition and consolidated into a hard permeable mass. Typically, the resin composition coated proppant is deposited in the fractures after a larger quantity of uncoated proppant material has been deposited therein. That is, the last portion of the proppant deposited in each fracture, referred to in the art as the "tail-in" portion, is coated with a hardenable resin composition. Upon the hardening of the resin composition, the tail-in portion of the proppant is consolidated into a hard permeable mass having a compressive strength in the range of from at least about 50 psi to 200 psi or more.
While the consolidated tail-in portion of proppant can be effective in preventing proppant flow-back with produced fluids if it is placed in the fractures near the well bore, very often the resin composition coated tail-in portion of the proppant is carried over uncoated proppant which previously settled near the well bore. This causes the resin coated proppant to be deposited deeply inside the fractures whereby it is incapable of preventing the flow-back of uncoated proppant between it and the well bore. Thus, there is a need for improved methods of placing proppant in subterranean zones whereby the flow-back of proppant with produced fluids is effectively prevented.